If it's regulated there may be a pot on the control card to set output. If it's unregulated or the ferroresonate type then you will have to either reduce the input AC voltage or deal with reducing the volts in the DC section. Regulated supplies with active components (pass transistors) will have a voltage divider across the output to feedback a portion of the voltage. Sometimes ther eis a pot in the chain but if not you might can replace one of the resistors with a pot of the same or higher value to trim the setting. It's hard to say without seeing the unit or a schematic.
Since you just need to shed 5 volts (might want to make it 7 or 8 in case the line voltage goes up) and the total current is not too high there are several alternatives but we need to know a little more about the supply.